There is no single, universally accepted definition of the range of photon energies which constitute X-rays. However, many skilled in this technology field use the following definitions: EUV (Extreme Ultraviolet) can cover the range of wavelengths from about 100 nm to about 10 nm; X-ray can cover the range of wavelengths from about 10 nm to about 0.01 nm. Soft X-rays, a subset of X-rays, can cover the range of wavelengths from about 10 nm to about 0.1 nm. There is a wide range of applications for radiation in the EUV and X-ray spectral ranges.
For wavelengths shorter than approximately 110 nm, there is a lack of viable materials which can be used to fabricate refractive optical elements for applications utilizing the EUV and X-ray spectral ranges. This is due to the fact that all materials absorb significantly at these wavelengths, particularly at thicknesses great enough to form a practical lens element. Therefore, reflective or diffractive optical elements are typically used for wavelengths of radiation shorter than approximately 110 nm. Such reflective elements can range from simple, planar mirrors to more complicated forms such as ellipses, parabolas, and combinations thereof. The ranges of wavelengths which require reflective optics therefore can include both the EUV range and the X-ray range.
As the wavelength of the radiation becomes shorter, the requirement on surface roughness for viable optical elements becomes correspondingly stricter as well. A complex relationship exists between the wavelength of the radiation, the angle of incidence of the radiation, the roughness of the reflective surface and the corresponding reflectivity of the incident radiation off of the surface. This can be seen from the results of sample numerical calculations, as shown in FIGS. 1A-1D, which are two-dimensional plots 110-140 illustrating reflectivity versus photon energy for copper surfaces of varying roughness and for different incident angles. The plot 110 illustrates reflectivity versus photon energy for an incident photon angle of 1 degree and surface roughness of 1 nm. The plot 120 illustrates reflectivity versus photon energy for an incident photon angle of 1 degree and surface roughness of 10 nm. The plot 130 illustrates reflectivity versus photon energy for an incident photon angle of 5 degree and surface roughness of 1 nm. The plot 140 illustrates reflectivity versus photon energy for an incident photon angle of 5 degree and surface roughness of 10 nm. As FIGS. 1A-1D illustrate, for high reflectivity it is necessary to have an appropriate combination of shallow angle of incidence and low surface roughness (low relative to the wavelength being reflected).
A surface can be brought to a very low roughness level through the use of machining techniques and/or polishing. Diamond-turning, which can involve the use of a specialized lathe combined with cutting tools utilizing a diamond cutting edge, can provide surface roughness as low as 1 nm. However, this can be achieved only in limited circumstances, having to do with the material and geometry of the part being fabricated. Polishing can also be employed to provide a desirable final surface roughness. However, the ability to effectively polish a surface is also dependent on the geometry of that surface. As a general rule, surfaces that are concave with a high degree of curvature are typically more difficult to fabricate to a very low roughness value than those which are flat to convex and have a low degree of curvature.
Synchrotrons can provide one flexible source of radiation in both the EUV and X-ray spectral ranges. Synchrotrons are typically part of a large, relatively expensive facility, usually supported by a governmental agency. The radiation from a synchrotron beamline typically is emitted in a very bright, narrow beam. Therefore, focusing optics, such as zone plates described below, can be effectively used as both collection and imaging elements over the EUV and soft X-ray ranges. Applications utilizing synchrotron radiation in the EUV and X-ray spectral ranges and zone plates for focusing can include soft X-ray biological microscopes and EUV exposure studies for semiconductor lithography applications.
One source of EUV and X-ray radiation that can be used as an alternative to synchrotrons are plasma based sources. Plasma-based sources can use either a high power pulsed laser system to generate the high temperature plasma required to generate these wavelengths, or they can use a pulsed electrical discharge. As an example, Energetiq Technology, Inc. of Woburn, Mass., offers for sale an EUV and soft X-ray source based on the use of a z-pinch technology that inductively couples pulsed dc energy into a discharge region, such that the required high temperature discharge can be attained to generate both EUV and soft X-ray radiation. As an example of the size of a discharge produced plasma (DPP) source, the z-pinch source from Energetiq Technology can produce an EUV and X-ray emitting spot that is approximately 0.4 to 1.0 mm in diameter.
When a DPP radiation source is used in place of a synchrotron radiation source, use of the condenser zone plate becomes less favorable. Useful zone plate throughput is limited theoretically to <20% for light incident within the small acceptance numerical aperture (typically less than 0.02 in the soft X-ray region). In a synchrotron-based system, enough power is available that a 90% (or more) loss of throughput may be acceptable. However, a DPP radiation source appropriate to a small laboratory will have limited output power and such losses would be unacceptable. Therefore a higher throughput condenser lens element is desirable when a DPP radiation source is used. There can also be instances where a higher throughput condenser lens element would be desirable for a synchrotron or other type of source as well.
An additional feature of the DPP radiation source (as compared to a laser plasma source) is that the size of the X-ray emitting region is relatively large. This allows use of a de-magnifying optic which concentrates the larger source size, providing higher illumination intensity while still allowing an adequate illuminated field of view. In addition, the larger source size relaxes the mechanical alignment and positioning constraints on the condensing optic.
One class of optical elements that can be used as an alternative to a condenser zone plate consists of grazing incidence reflective devices. These are reflective elements configured such that the angle of incidence of the light to be focused is small—typically only a few degrees or less. By keeping the incidence angle small and the surface roughness very low, the throughput of grazing incidence devices can be quite large—in excess of 50%, and approaching 100% for some configurations.
Grazing incidence devices can be used in many possible configurations (e.g., Wolter, de-magnifying or magnifying ellipse, tandem ellipse (unity magnification), capillaries). Grazing incidence devices can achieve high throughput (>50%), and are robust and rugged due to their macroscopic size. However, it can be difficult to machine small, high aspect ratio grazing incidence devices.
Zone plates can use a non-uniform, circular transmission grating to diffract radiation. Transmission efficiency (throughput) of zone plates are approximately 20% or less. In addition, zone plates are microscopic, fragile and expensive to fabricate, and require very specialized manufacturing facilities. Furthermore, zone plates can suffer from severe chromatic aberration, while reflective optical elements are generally achromatic.